How to change the output voltage from 13.8V to 12v with this circuit

[legarem]

Hello

I have a Pyramid power supply which gives 13.8V . I adjusted the pot inside the PS but can get near 13.2volts at lower point. I want 12 volts to feed a Mac mini. This circuit seems interesting because it uses discrete parts instead of ubiquitous 78xx with a pass transistor. Any other mods suggestion ? Thanks

 

[Elvee]

It's just a bog-standard, ordinary reg.
To reduce the output voltage, replace ZD1 with a 10V one, and you will reach the desired range

 

[legarem]

Thanks. I thought I could replace R7 or R10 to get 12 V.

 

[Elvee]

R10 would need to be zero, which is unsafe, and there is an unavoidable transistor Vbe anyway.

 

[stocktrader200]

R10 try 680 or 430 ohm.

 

[NareshBrd]

Enough amps for the Mini?
What happened to the original supply?
Compare the current ratings, 3055 should give enough juice.

 

[jan.didden]

R10 try 680 or 430 ohm.

You'll never get down to 12V with a 12V zener.

Jan

 

[legarem]

The Mac mini takes no more than 2 amperes so this power supply can feed it. For audio, when you use this computer you would not believe the sound you will get when you change the SMPS to a linear PS. It is simply NOT the same machine.

 

[Ketje]

Best to skip that part, can only do harm. Looks like a left-over from a supply that needs a start-up help.
Mona

 

[jan.didden]

Yes, I noted that too. Maybe it really does need a start-up circuit.
Some experimental versions of the superreg needed it too.
Happens sometimes if the reference is fed from the output.
Does not make it a bad supply though.

Jan

 

[TNT]

The Mac mini takes no more than 2 amperes so this power supply can feed it. For audio, when you use this computer you would not believe the sound you will get when you change the SMPS to a linear PS. It is simply NOT the same machine.

On using what interface do you detect this sound difference?

//

 

[legarem]

I took out the original SMPS from the Mac mini and replaced it with a cap filter bank. The linear PS is outboard. I use the Mac mini with an an Audio gd DI-20 Usb to SPDIF or I2S that goes to the dac.

 

[jan.didden]

So it's all digital then. If you really hear a difference, get a competent DAC.

Jan

 

[legarem]

A competent dac !!! You are kidding. I have a DCS Elgar Plus with a Purcell. If you know this dac you know it's More than competent even if it is a little aged.

 

[jan.didden]

Well if you hear a difference with the MAC power supply then this DAC is not competent.
You can't fool physics.

Jan

 

[Elvee]

Best to skip that part, can only do harm. Looks like a left-over from a supply that needs a start-up help.
Mona

It doesn't look like a startup circuit, the opposite in fact: it ensures a clean cut as soon as the AC power is removed.
Might be useful for loads that don't like a brown-out condition?

 

[Bill_P]

Here is the LTspice simulation.

 

[jan.didden]

Did you try to run it with that circuit disconnected?

Jan

 

[Bill_P]

Small change in start-up with input circuit disconnected.

 

[Ketje]

As long as the saturation voltage of Q1 + voltage drop on D8 is more then the Vbe of Q2 nothing happens.
The R2 gives positive feedback, so if someting goes wrong the output jumps to the max possible voltage.
Mona